In the past, a disk array system such as a RAID (Redundant Array of Inexpensive/Independent Disks) system has included a mechanism that maintains redundancy.
For example, when one of the disks included in a RAID group becomes unavailable due to a failure and the redundancy is lost, the RAID system reconstructs data in a hot spare disk instead of the unavailable disk. Thus, the redundancy recovers (for example, refer to Japanese Laid-open Patent Publication No. 07-098635). The processing is generally called rebuilding processing.
In order to perform the rebuilding processing, the disk to be rebuilt is determined as follows:
(1) If a hot spare disk that has been dedicatedly assigned to a RAID group having a disk failure exists, the RAID system handles the hot spare disk as the target of the rebuilding processing.
The hot spare disk dedicatedly assigned to a specific RAID group is called a “dedicated hot spare disk” hereinafter.
(2) If no dedicated hot spare disks exist but a hot spare disk that can be assigned to an arbitrary RAID group exists, the RAID system handles the hot spare disk as the target of the rebuilding processing.
The hot spare disk that is available to an arbitrary RAID group is called a “global hot spare disk” hereinafter.
After determining the target of the rebuilding processing in this way, the RAID system performs the rebuilding processing on the hot spare disk determined as the target of the rebuilding processing.
The disks included in a RAID group have unique characteristics. For example, the disks have characteristics determined by various factors such as the numbers of rotation, the transfer rate, and an algorithm to be used in firmware for disk control.
Thus, when a disk having a different characteristic from the failing disk as a hot spare disk is used, there is a possibility that the characteristics before and after the occurrence of the failure of the entire RAID group may differ.
For example, there is a possibility that the responses by the RAID group may extremely differ between before a failure of a disk and after the rebuilding processing on the disk. Currently, from the viewpoint of costs, a dedicated hot spare disk can be limitedly assigned to only one RAID group.
For that reason, when a RAID group includes disks having a different characteristic from those in other RAID groups, for example, a dedicated hot spare disk may be required for each of the RAID groups to solve the problem that the characteristic of the RAID group may change.
When a RAID group already using a dedicated hot spare disk has a disk failure, the RAID group uses a global hot spare disk 220.
Since a global hot spare disk 220 is available to an arbitrary RAID group, there is a possibility that the global hot spare disk 220 having the most proper characteristic to the RAID group having a disk failure may have already been used by another RAID group.
Under this circumstance, it is difficult to solve the problem that the characteristic of the RAID group may change. Defining many dedicated hot spare disks for each RAID group can address the problem that the characteristic of the RAID group may change. However, in this case, the probability that all of the dedicated hot spare disks are used is low. This causes the problem that the utilization ratios of the disks are lowered.